1. Field of the Invention
This invention is concerned with manufacture of high grade viscous oil products from crude petroleum fractions. It is particularly directed to the manufacture of high quality lube basestock oils from crude stocks of high wax content, commonly classified as "wax base" as compared with the "naphthenic base" crudes. The latter crudes are relatively lean in straight chain paraffins and yield viscous fractions which inherently possess low pour points. More specifically, the invention is concerned with improving the viscosity index of catalytically dewaxed lube basestock oils.
2. Description of the Prior Art
High quality lube basestock oils are conventionally prepared by refining distillate fractions or the residuum prepared by vacuum distilling a suitable crude oil from which the lighter portion has been removed by distillation in an atmospheric tower. Thus, the charge to the vacuum tower is commonly referred to as a "long residuum" and residuum from the vacuum tower is distinguished from the starting material by referring to it as the "short residuum".
The vacuum distillate fractions are upgraded by a sequence of unit operations, the first of which is solvent extraction with a solvent selective for aromatic hydrocarbons. This step serves to remove aromatic hydrocarbons of low viscosity index and provides a raffinate of improved viscosity index and quality. Various processes hve been used in this extraction stage, and these employ solvents such as furfural, phenol, sulfur dioxide, and others. The short residuum, because it contains most of the asphaltenes of the crude oil, is conventionally treated to remove these asphalt-like constituents prior to solvent extraction to increase the viscosity index.
The raffinate from the solvent extraction step contains paraffins which adversely effect the pour point. Thus, the waxy raffinate, regardless of whether prepared from a distillate fraction or from the short residuum, must be dewaxed. Various dewaxing procedures have been used, and the art has gone in the direction of treatment with a solvent such as methyl ethyl ketone/toluene mixtures to remove the wax and prepare a dewaxed raffinate. The dewaxed raffinate may then be finished by any number of sorption or catalytic processes to improve color and oxidation stability.
The quality of the lube basestock oil prepared by the sequence of operations outlined above depends on the particular crude chosen as well as the severity of treatment for each of the treatment steps. Additionally, the yield of high quality lube basestock oil also depends on these factors, and as a rule, the higher quality sought, the less the yield. In general, naphthenic crudes are favored because less loss is encountered, particularly in the dewaxing step. In many cases, however, waxy crudes are more readily available, and it would be desirable to provide a process for preparing high quality lube basestock oils in good yields from such waxy crude oils.
In recent years techniques have become available for catalytic dewaxing of petroleum stocks. A process of that nature developed by British Petroleum is described in the Oil and Gas Journal dated Jan. 6, 1975, at pages 69-73. See also U.S. Pat. No. 3,668,113.
In U.S. Pat. No. Re. 28,398 is described a process for catalytic dewaxing with a catalyst comprising zeolite ZSM-5. Such process combined with catalytic hydrofinishing is described in U.S. Pat. No. 3,894,938 for reducing the pour point of a sulfur and nitrogen containing gas oil boiling within the range of 400.degree.-900.degree. F.
In U.S. Pat. No. 3,979,279 a stabilized lubricating oil stock resistant to oxidation and sludge formation upon exposure to a highly oxidated environment is formed by contacting a high viscosity lubricating oil stock with hydrogen in the presence of a catalyst of low acidity comprised of a platinum-group metal on a solid refractory inorganic oxide support.
A two-stage process for preparing a high quality lube basestock oil is disclosed in U.S. Pat. No. 4,181,598 in which a raffinate is mixed with hydrogen and the mixture contracted with a dewaxing catalyst comprising a ZSM-5 type catalyst to convert the wax contained in the raffinate to low boiling hydrocarbons and subsequently, contacting the dewaxed raffinate in the presence of hydrogen at a temperature of 425.degree.-600.degree. F. with a hydrotreating catalyst comprising a hydrogenation component on a non-acid support. Hydrotreating the dewaxed raffinate is limited to saturate olefins and reduce product color without causing appreciable desulfurization.
It has been found in a hydrofinishing optimization study that at higher temperatures above 500.degree. F. oxidation stability as measured by RBOT declined, but that the viscosity index could be increased several numbers. Also, it has been discovered by the inventor's coworkers that high V.I. lubes can be manufactured from marginal crudes by successive furfural extraction, catalytic dewaxing, and hydrotreating over strong hydrogenation catalysts to desulfurize and crack the sulfur compounds to lower boiling product out of the lube range as disclosed in copending U.S. patent application Ser. No. 528,331 filed concurrently with the present application.
It is an object of this invention to provide a process for increasing the viscosity index of a catalytically dewaxed lube basestock oil under conditions which greatly reduce the sulfur content of the lube oil basestock without loss of lube yield.
Another object of the invention is to produce a high V.I. lube oil basestock from catalytically dewaxed lube fractions to a viscosity index comparable to that achieved by solvent dewaxing. Other objects will be evident to those skilled in the art upon reading the entire contents of this specification, including the claims thereof.